1. Field of the Invention
This invention relates to an apparatus and process for providing a homogeneous mixture of two gaseous fluids. More particularly, this invention relates to an apparatus and process for providing a homogeneous mixture of a gaseous fuel and oxidant, preferably air, to a combustor.
2. Description of Prior Art
Premixing of fuel and air is becoming more and more attractive as one of the most effective methods for reducing NO.sub.x formation in combustion processes. As the fuel/air ratio changes within a combustor, the NO.sub.x formed by the combustor changes due to variation in the peak flame temperature and the availability of oxygen as the fuel/air ratio is altered. Premixing of fuel and air minimizes the formation of pockets of higher flame temperatures and oxygen availability within the flame, both of which promote higher NO.sub.x formation. In general, premixing also intensifies combustion and, in certain burner designs, enhances internal combustion products recirculation, thereby providing stable combustion from highly fuel-lean to highly fuel-rich conditions. To achieve the best results, it is desired to mix the fuel and air as completely as possible prior to ignition.
Numerous mixers and mixing devices are known for premixing fuel and combustion air. Mixing devices for this purpose are generally grouped in two classifications: manual mixers which require manual adjustments to maintain a desired fuel/air ratio as rates of flow are changed, and automatic mixers which automatically maintain within their rated capacity a substantially constant fuel/air ratio over a range of flow rates. Within the classification of automatic mixers are gas jet mixers which utilize the kinetic energy of a jet of gas issuing from an orifice to entrain all or part of the air required for combustion, air jet mixers which utilize the kinetic energy of a stream of air issuing from an orifice to entrain the gas required for combustion, and mechanical mixers which utilize mechanical means to mix gas and air, neglecting entirely any kinetic energy in the gas and air, and compressing the resultant mixture to a pressure suitable for delivery to its point in use.
In general, mixer designs without any internal obstructions are termed "open" mixers. These mixers require a long downstream region for complete mixing, thereby creating a large quantity of combustible mixture which, in turn, increases the potential for flashback and/or explosion.
Accordingly, to enhance mixing in a much shorter downstream region, there also exist mixer designs which utilize internal baffles and diverters. However, the major disadvantage of these designs is the repercussion of flashback. If flashback were to occur, the flame front would travel upstream of the internal baffles and then be trapped.
Finally, in either mixer design, no means for compensation of non-uniform or skewed profile air flow conditions which are common in real applications is provided. As a result, there exists a need for rapid mixers having no downstream restrictions and having the capability of compensating for non-uniform or skewed air flow profiles.
U.S. Pat. No. 3,718,426 to Harris teaches a burner comprising a bundle of tubular elements, each of which is open at each of its ends, for the supply of combustion air to the burner. Each of the tubular elements is provided at one end with a head portion having an external cross-section such that when the bundle of tubular elements are bunched together with their head portions in contact with each other, fuel orifices are formed between the head portions for fuel supply to the burner. The burner further comprises a housing which defines a fuel chamber and a fuel inlet for introducing fuel into the fuel chamber. The bundle of tubular elements are bunched together in the fuel chamber in a laterally spaced, substantially parallel relationship. The back wall of the housing is provided with a plurality of circular openings for receiving the circular non-headed ends of the tubular members and the front wall of the housing is provided with a hexagonal opening in which the enlarged head portions of the bunched tubular elements are located in contact with each other, the head portions of the outer ring of the tubular elements being in close contact with the edges defining the hexagonal opening. As fuel is fed into the fuel chamber, it circulates around the tubular elements and passes out of the chamber through the fuel orifices formed between the head portions of the tubular elements. When the fuel issuing from the fuel orifices is ignited, the burning fuel causes combustion air to be drawn through the tubular elements by induction to mix with the issuing gas or other fuel to produce the required flame. Accordingly, ignition of the fuel causes the induction of combustion air through the tubular elements for mixing with the fuel issuing from the fuel chamber.
U.S. Pat. No. 4,100,733 to Striebel et al. teaches an apparatus for supplying fuel to the combustion chamber of a gas turbine engine comprising a fuel chamber defined by an upstream wall and a downstream wall through which a plurality of mixing tubes extend to flow air through the fuel chamber. Each of the primary mixing tubes is in communication with the fuel chamber through fuel orifices formed by the primary mixing tubes. Fuel, under pressure, is introduced from the fuel chamber through the fuel orifices into the mixing tubes in which it mixes with the air flowing therethrough.
U.S. Pat. No. 5,361,586 to McWhirter et al. teaches a gas turbine combustor having a plurality of concentrically arranged annular passages, each having an inlet and a discharge end, a first fuel discharge port for each of the annular passages for introducing a fuel therein, and means for separately controlling the introduction of fuel into each of the annular passages through its fuel discharge port. In accordance with one embodiment, means for separately controlling the introduction of fuel into each of the annular passages through its respective fuel discharge port comprises an approximately toroidal manifold for each of the annular passages, each of the toroidal manifolds being disposed upstream of its respective annular passage.
U.S. Pat. No. 4,967,561 to Bruhwiler et al. teaches a gas turbine combustion chamber having an air distribution chamber and a combustion space locationally separated from one another within a combustion chamber shell. A plurality of tubular elements is located between the distribution chamber and the combustion space in which elements of premixing and pre-evaporation of fuel oil supplied through premixing nozzles and/or a premixing of a fuel gas supplied through the premixing nozzles takes place with compressor air. Each tubular element is provided with a flame holder in the direction of the combustion space. A diffusion nozzle for fuel directed into the combustion space is located within the flame holder. In operation, only a small part of the fuel supplied to each element is burned by means of the diffusion nozzle, the major proportion, on the other hand, being burned by means of the premixing nozzles.
U.S. Pat. No. 4,887,963 to LeMer teaches a gas burner comprising an air box, one wall of which is perforated with a large number of closely spaced orifices, a gas feed tank connected to a pressurized fuel gas source, and a plurality of hollow needles, each connecting the inside of the feed tank to the central zone of the inlet of an orifice in the perforated wall so as to define within this orifice one of a plurality of flame production sites. The air box is connected to a pressurized air source, the orifices are cylindrical, and a mechanical obstacle is provided in the center of the outlet of each orifice for deflecting the gas jet leaving the needle and mixing it with the airstream which surrounds it.
U.S. Pat. No. 4,455,840 to Matt et al. teaches a ring burner for a ring combustion chamber divided into a large number of honeycomb-like parallel axis canals for the combustion air, by radial and circumferential plate canals, or by radial plate canals, longitudinal tubing, radial tubing and annular tubing into which combustion gas is introduced from nozzles in the surrounding walls. At the burner outlet, fuel retention nozzles are provided above the frontal surface area of the plate canals or tubes. The fuel nozzles are provided in front of the burner inlet for operation as a dual-fuel burner with gaseous and liquid fuels.
U.S. Pat. No. 4,378,206 to Kullendorff et al. teaches a fluidized bed combustion chamber having a perforated bottom plate into which air nozzles are pressed in a manner similar to that used for tubes in a tube sheet of a heat exchanger. About one-third of the nozzles are connected to a start-up or auxiliary combustion chamber for blowing in hot gas for heating the bed to the flash-point or ignition temperature of the fuel.
U.S. Pat. No. 4,189,294 to Rice et al. teaches a flameless combustion burner having an ignition zone, a catalyst zone, and a plenum. To obtain flameless combustion, a fuel and a combustion supporting gas are introduced into the ignition zone from the plenum and a flame is initiated by means of a starting device. The products of combustion flow over the catalyst to heat it to a temperature at which it can initiate a flameless combustion.
U.S. Pat. No. 689,814 to Dow teaches a burner having a gas chamber provided with burner tubes, a plurality of mixing tubes discharging into the gas chamber, automatic or self-closing valves operably linked to the mixing tubes to prevent backflow from the gas chamber, means for introducing gaseous fuel under pressure through the mixing tubes, and means for independently controlling the inflow of fuel in connection with the respective mixing tubes.